The present invention relates to a method for growing up single crystals and especially to the growth of single crystals of highly active materials like Al or Al-Li.
High quality single crystals having low dislocation density and controllable ingredients play an important role in scientific researches and engineering applications. Generally, methods for growing single crystals include:
1) Czochralski crystal pulling technique, PA0 2) Floating zone crystal growing technique, PA0 3) Bridgman crystal growing technique, and PA0 4) Directional freezing crystal growth technique, etc. PA0 1) Using thermodynamically stable materials for ceramic shell mold, like Al.sub.2 O.sub.3, ZrO.sub.2, ZrSiO.sub.4, etc.; or PA0 2) Applying proper coatings on the shell mold for isolating the melted metal from the shell mold. For instance, graphite, silicon carbide (SIC), zirconium oxide (ZrO.sub.2), etc. are broadly used. PA0 a) preparing a single crystal selector wax component having an upper end; PA0 b) preparing a graphite mold base having a hole at the bottom of said graphite mold base, wherein the diameter of said hole is substantially identical to the diameter of said upper end of said single crystal selector wax component; PA0 c) filling wax into said graphite mold base, and, wax welding the said single crystal selector wax component to the wax filling at the said hole, to form an assembled wax component which has an graphite shell; and PA0 d) applying investment casting shell mold manufacturing processes on said assembled wax component to prepare a graphite mold with a single crystal selector, wherein said investment casting processes include dipping, firing and dewaxing, said graphite mold has a ceramic shell and an inner surface, said inner surface of said graphite mold in contact with growing up single crystals is of graphite.
For saving time and cost, the Bridgman crystal growing technique, the directional freezing crystal growth technique, or the modifications of the above two methods often apply in the growth of single crystal materials of metals, like a single crystal superalloy turbine blade.
In the Bridgman crystal growing technique and the directional freezing crystal growth technique, starting materials are maintained in vacuum or controlled atmosphere. By adjusting the temperature gradient of materials, or by unidirectional heat dissipation, liquid metal melted from the starting materials begins to be solidified. By means of a single crystal selector, the metal crystallizes to form a single crystal. Consequently, single crystal selectors are of great importance in the above two methods. The single crystal selectors can be of different types include conical type, neck type, multiple neck type, S-shape type, Z-shape type, spiral type, etc. Due to the complexity of the shape of single crystal selectors, molds used in single crystal growth are typically investment casting shell molds manufactured from ceramic powder (SiO.sub.2 or Al.sub.2 O.sub.3) with proper binding materials.
When the single crystals grow up, the shell mold made of ceramic powder will be in contact with the melted metal for a long time. In particular, the ingredients of the shell mold may have bad effects on the quality and purity of the growing single crystal. When an active material grows, such as pure aluminum or aluminum alloy or magnesium alloy, the ingredients of the shell mold will react on the melted metal. As a result, the ingredients of the single crystal can hardly be controlled, and this will promote the nucleation of new grains, which is detrimental to the growth of a good single crystal. Although there are still few chances to obtain a single crystal, its dislocation density is too high to be a high quality single crystal.
The methods used for solving this problem include:
These methods also have deficiencies. Ceramic powder and coatings are mixed with binding materials when they are applied to a mold. The binding materials are usually selected from the following materials: water glass (sodium silicate), silica gel solution, or ethyl silicate. After baking, there are still residue of the binding materials. The residue will react with the melted metal, which causes spalling and failure of the coating. Additionally, when the coating material is applied to the shell mold, single crystal selectors having narrow hole(s) are easily choked by the coating material, then single crystals can not grow.
In the growth of highly active materials, like pure aluminum or aluminum alloy or magnesium alloy, graphite had been found to be an ideal passive crucible material. In the other hand, commercially supplied large graphite pieces can be easily machined. Consequently, graphite had been turned into crucibles on a lathe for growing single crystals. Because of the limitation of a lathe, graphite can only be turned to form a conical cavity at the bottom of the crucible for selecting single crystals. However, a conical type single crystal selector is not a good selector, because many grains could be nucleated, and it does not have the ability of selecting a single crystal in the optimum growing direction. Therefore, the effect of such a selector is not satisfactory.